The invention concerns a direct energy-transfer converter of the AC/AC or DC/DC type, consisting of a self-regulated synchronous rectifier. In the remainder of this document, the expression “synchronous rectifier” preferentially refers to a self-regulated synchronous rectifier, meaning a rectifier in which the rectifying switches are controlled over one or more transformer windings.
Direct energy transfer conversion systems are known which include an input voltage source and at least one controlled primary switch, driving a transformer whose secondary is fitted with at least two power switches constituting a self-regulated synchronous rectifier. Said synchronous rectifier is mounted in cascade with a filter delivering a continuous controlled voltage into an application. In this type of conversion chain, the purpose of the rectifier is:                to deliver to the application, through the filter, the energy transferred by the transformer during the conducting period of the primary switch, called the direct transfer phase, and        to block the transfer during the non-conducting period of said primary switch, the application being supplied by the coil of the filter through a freewheel switch, such as a MOSFET, from the rectifier, during this non-conducting period of the main primary switch, called the freewheel phase.        
The secondary part (100) of such a converter of the asymmetrical direct energy transfer type, is represented on FIG. 1. Rectification is said to be of the “active clamp” type.
Part 100 includes:                a secondary winding (10) of the power transformer,        a synchronous rectifier (1) of the direct transfer type,        an LC output filter (5).        
The synchronous rectifier (1) includes two secondary power transistors (20 and 30) and their respective control elements (21–22 and 31–32), arranged so as to ensure direct transfer of energy via the transistor (20) and the freewheel phase implemented by transistor 30, with the minimum of losses. Elements 21 and 31 are a diode in parallel with a capacitance, for example, and elements 22 and 32 are Zener diodes, for example.
This solution is possible only by optimisation of the gate control voltages. The wiring of the self-regulated control elements results in voltages on the gates of the power transistors whose amplitudes are directly dependent on the input voltage to the terminals of the primary winding (not shown) and therefore the output voltage at the terminals of the secondary winding (10). In asymmetrical direct energy transfer systems of the active clamp type, the amplitudes of the gate voltages vary in the opposite sense. For example, the voltage available at the gate of transistor 20, in the direct phase, is maximum for a maximum input voltage. Conversely, when the input voltage reduces, the voltage available at the gate of transistor 30 increases, to reach a maximum value that may be unsuitable.
The result is that this solution gives rise to a certain number of difficulties. It can happen, in fact, that the maximum gate voltage of one of the two transistors is not sufficient to effect control. This is the case in particular when the voltage at the terminals of the secondary winding (10) is low, at about 2V. A low voltage at the terminals of the secondary winding can, in particular, occur in the case of a variable input voltage at the terminals of the primary winding, of 36V and 72V, for example, for an average voltage of 48V, or between 18V and 36V for an average voltage of 24V, and even more so for wider variations of input voltage, such as 18V–72V. The voltage at the terminals of the secondary winding can then be just sufficient to control transistor 20 in the direct phase, but insufficient to control transistor 30 in the freewheel phase.
Conversely, for high output voltages, above 10V, it is not easy to find a good compromise between an adequate gate voltage on the direct transistor, over the whole input voltage range, and maximum gate voltage but less than the maximum value allowable for the gate of the freewheel transistor.